HR: 1330h
AN: S42A-0150    [PDF]
TI: Crustal Velocity Structure From First Arrival Seismic Tomography in the Strait of Georgia, British Columbia and Washington
AU: * Dash, R K
EM: rdash@uvic.ca
AF: School of Earth and Ocean Sciences, University of Victoria, Victoria, BC V8W 3P6 Canada
AU: Spence, G D
EM: gspence@uvic.ca
AF: School of Earth and Ocean Sciences, University of Victoria, Victoria, BC V8W 3P6 Canada
AU: Riedel, M
EM: mriedel@pgc-gsc.nrcan.gc.ca
AF: Pacific Geoscience Centre, Geological Survey of Canada, Sidney, BC V8L 4B2 Canada
AU: Hyndman, R D
EM: rhyndman@nrcan.gc.ca
AF: School of Earth and Ocean Sciences, University of Victoria, Victoria, BC V8W 3P6 Canada
AU: Hyndman, R D
EM: rhyndman@nrcan.gc.ca
AF: Pacific Geoscience Centre, Geological Survey of Canada, Sidney, BC V8L 4B2 Canada
AU: Brocher, T M
EM: brocher@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, M/S 977,, Menlo Park, CA 94025 United States
AU: Pratt, T L
EM: tpratt@usgs.gov
AF: U.S. Geological Survey, School of Oceanography, Box 357940, Univ. of Washington, Seattle, WA 98195 United States
AB: Seismic refraction/wide-angle reflection data were collected in May 2002 in the Strait of Georgia, southwestern British Columbia and northwestern Washington. The experiment, part of the Georgia Basin Geohazards Initiative, provides knowledge about potential geohazards associated with near-surface crustal faulting in the strait. The last significant earthquake (M = 4.6 in 1997) within the strait reflects the regional seismic hazard. The seismic survey was aimed at studying the three-dimensional (3D) velocity structure of the shallow upper crust particularly the Cenozoic sedimentary basins, the Outer Island fault and the Lummi Island fault. The data were acquired using 48 Reftek stations deployed onshore to record the signals from a 120 cu. in. airgun fired at a nominal interval of 5 sec or 12 m. Single channel reflection data were collected simultaneously. The experiment provided useful data from 42 of the 48 stations deployed. The data quality at many receivers was very good. The average maximum source-receiver offset for first arrivals was 17 km with a maximum offset of 35 km. Preliminary 2D velocity modeling indicates three layers in the shallow upper crust down to a maximum depth of 3 km. The uppermost layer in the strait represents recent unconsolidated sediments. The second layer consists of basin fill sediments (velocity 4.1- 4.4 km/s) associated with the Upper Cretaceous Nanaimo Group. These sediments occur at the surface within the Canadian Gulf Islands but at a depth of about 700 m within the strait. The underlying basement layer (velocity 5.3 - 5.7 km/s) at a depth of 1.7 km possibly represents the volcanic Wrangellia terrane, extending from Vancouver Island beneath the strait. The wide distribution of receivers in the islands and shots in the surrounding waters provides excellent constraints for 3D inversion of first arrival travel times and hence for a high-resolution 3D image of the shallow structure.
DE: 7200 SEISMOLOGY
DE: 7294 Instruments and techniques
DE: 8180 Tomography
SC: Seismology [S]
MN: 2003 Fall Meeting